In reply to Ramblin dave:
In the early days of the new format, near the final, they would work in professional kitchens. I remember a barrister on there saying he want to win so he could have a career change to being a chef. Now you have to assume he's earned a decent amount and maybe doesn't need the money - fair enough.
If I decided to quit a cook, firstly I don't see why I'd want to turn a fun hobby into my source of income, but if I did I'd want to cook in my own small restaurant, not sweat under the pressure from some shouty chef.
My wife and I visit a little restaurant run by an ex lawyer or accountant, he cooks a few nights a week for a few covers. Food is okay, bug portions, great flavours, but it's only excellent home cooking. It's a far cry from the imagination you get at somewhere in the Michelin or Good Food guides.
I don't understand the motivation behind some of them, and can't help but imagine great disappointment. Those that have left to write books or teach courses have done very well for themselves. But unless you build enough of a brand (eg Hairy Bikers, Huw F-W, Gordon Ramsay) you're going to be just a passing phase.